Film testing: help..........

[Peter Coats]

I am trying to muddle my way through determining my EI for TMax 100, as well as development times. Happily, I have access to a both densitometer and copy table at my local university.

What I am doing is placing a grey card on the table and lighting it with bulbs on either side at 45 degree angles. I then use the in-camera meter (Mamiya 6, shooting 120) for exposure data, and expose the film placing exposures in Zones 0 through X.

My problem seems to be this: the meter indicates the same exposure for the grey card at film speeds 64 through 100: f5.6 @ 1/125. Only when I dial in ASA of 50 does the meter change, indicating f5.6 @ 1/60. My corresponding Zone I densities are .06 -.08 above fb+f when ASA is between 64-100. However, when I set the ASA to 50, my Zone I densities jump to 0.14.

From my research, a density of .10 - .12 above fb+f seems appropriate for Zone I, as I will be printing on VC paper on a diffusion enlarger.

Unfortunately, my choice seems to be exposures that will give either .06 -.08 or .14 for Zone I - no in-between. I have yet to get out in the field and expose rolls at either of my two choices and see which gives more manageable negatives, and perhaps that is the only solution. But I wanted to call on the expertise of the members......am I missing something?

Any suggestions?

Thanks in advance

 

[john_s]

You're on the right track, but your measuring instrument is not making it easy for you.

For all its handling and optical qualities, the Mamiya 6 has a meter that, while generally ok, is not well suited for this task because it's not designed to display readings at fractional stop values. The meter is not through the lens and has a fairly wide angle of acceptance. On Auto Exposure, it will set the shutter to use in-between values but it will only display one of the standard speeds (1/125, 1/60 etc) in the viewfinder. On manual setting, you can set a shutter speed and try to approximate the exact in-between aperture by turning the aperture ring back and forth, but it's not very precise.

So, I recommend that you find a good spot meter. Check that it agrees with your Mamiya 6 meter on simple scenes, like a large uniformly lit wall. It should be very close. Then use it for setting manual exposures on the camera. Set your lens to infinity (to avoid the light fall off that occurs with lens extension). The tungsten light might behave a bit differently to sky light, but for the first round or two of testing would probably be ok. (Can someone confirm this?)